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Unnamed U.S. officials told NBC News the attack was "clearly the work of a state actor."

Russia is behind a spearphishing attack that forced the Pentagon to shut down its Joint Staff unclassified email system, NBC News is reporting.

The hack, which took place around July 25, affected approximately 4,000 military and civilian personnel, NBC News reported. The news outlet cited U.S. officials who called the intrusion a "sophisticated cyberattack."

Pentagon officials have kept the system offline for almost two weeks.

According to the NBC report, the cyberattack used a very fast-paced automated system that collected large amounts of data from the email accounts and then distributed it to thousands of other accounts.

Officials told NBC News the hackers used encrypted social media accounts to perform the hack.

Unnamed U.S. officials told NBC News the attack was "clearly the work of a state actor."

A Defense Department spokeswoman would not confirm or deny to Nextgov that Russia was behind the intrusion and would not answer questions beyond public statements made when the email system was taken down about 10 days ago.

CNN reported classified networks were not affected by the attack, and that senior officials had hoped the network would be restored by Thursday. NBC News reported it could be online by the end of this week.